It has long been appreciated in the aluminum industry that sound products and good operating economics require treatment of molten metal to reduce certain types of defects in the product made from the metal caused by impurities in the metal prior to casting the metal. This is especially true for ingots which are subsequently worked to produce wrought products. One impurity commonly encountered is gas entrapped or dissolved in the metal during its melting and transfer. The gas is primarily hydrogen probably generated by moisture contacting the aluminum while molten. Likewise oxygen is acquired on the surface of molten aluminum which oxidizes the aluminum quite readily. Upon solidification of the metal, a considerable amount of gas and oxide particles are trapped within the metal. In subsequent fabrication, such entrapped impurities develop voids or discontinuities within the fabricated product that create weak areas in the product. The problem becomes more acute in high strength aluminum devices where voids and discontinuities not only create areas of weakness but can give rise to further defects, as explained below, which may constitute sufficient cause to reject the devices.
Other impurities commonly present in aluminum are dissolved trace elements, e.g., sodium, calcium, and lithium. This is introduced in the smelting process or in remelting of scrap metal. While trace elements, in the amounts generally encountered in aluminum, may not create severe difficulties in the final product itself, even miniscule amounts of trace elements give rise to serious problems in rolling and other drastic working operations especially in alloys containing magnesium. For instance, as little as 0.001% sodium or calcium can cause very serious edge cracking in the hot rolling of aluminum slabs, containing 2 to 10% magnesium, in a reversing mill.
It has been found that if the sodium and calcium content can be reduced to 0.0002% or less and especially to 0.0001% or less, on a commercial rather than mere laboratory basis, marked improvements in hot rolling can be realized such that heavy reductions of 20% or more per roll pass at temperatures of about 750.degree. F. or more can be readily employed even on relatively thick stock without excessive edge cracking. In addition, such very low sodium and calcium levels foster increases of 20% or more in continuous casting rates for aluminum ingots.
Various methods have been proposed to reduce the oxide, trace elements, and gas content of molten aluminum and in this connection reference is made to U.S. Pat. No. 3,767,382, granted to Marshall Bruno et al and incorporated herein by reference, wherein a process is described in which molten aluminum is treated with selectively maintained salt flux in a compact system to decrease its oxide, gas, and trace elements. Gas removal is further aided by stripping with a non-reactive stripping gas. The system features an intensely agitated zone for contacting the metal and the salt flux followed by a quiet separation zone. Molten metal introduction, agitation, and flux characteristics are utilized to achieve required efficiencies.
U.S. Pat. Nos. 3,839,019 and 3,849,119 granted to Marshall Bruno et al and both incorporated hereby reference describe processes in which aluminum is purified by chloridizing a molten body of aluminum. High metal chloridization rates are achieved in a system wherein chlorine utilization efficiency is 100% or very closely approaches this level. The system includes a chlorine-metal contacting technique which includes an agitator and which controls and maintains contacting conditions to optimize efficiency.
U.S. Pat. No. 4,390,364 granted to Ho Yu and incorporated herein by reference describes a method of treating molten metal containing suspended particles typically comprising buoyant liquid such as liquid salt or suspended phases are treated to coalesce or agglomerate the particles so that they are more readily separated by gravity in the molten metal.
Each of these processes includes some provision for agitating or stirring a chlorinaceous fluxing gas in the molten metal to disperse the gas and thereby increase its surface area and effectiveness in removing impurities. These methods have achieved commercial success. However, lowering the gas and trace element content in aluminum alloys is very difficult.
One example of the difficulty in reducing the trace element content by chlorination is that the magnesium present in the aluminum alloy melt reacts simultaneously with the chlorine. This occurs even though chlorine, or the reaction product of chlorine with aluminum, aluminum chloride, react with sodium and calcium preferentially over magnesium at equilibrium conditions.
It is believed that chlorine released in the melt would first be expected to largely form aluminum chloride because aluminum is by far the major component in the melt. Next in sequence, some of the aluminum chloride may encounter and react with magnesium in the melt to form magnesium chloride because magnesium is usually more concentrated than the other melt components capable of reacting with aluminum chloride. Finally, if contact with the metal is maintained long enough, the magnesium or aluminum chlorides encounter the trace amounts of sodium and calcium and react to form the final equilibrium product, sodium, and calcium chlorides. The rate of chlorination and magnesium concentration are factors determining how far and how rapidly reaction proceeds through this sequence to the final equilibrium product, sodium and calcium chlorides.
At commonly used chlorination rates, final equilibrium is difficult to achieve without long contact times. Accordingly, it has been difficult to achieve extremely low sodium and calcium levels under commercial production plant conditions which require comparatively large amounts of molten metal to be treated rather rapidly.
In view of the foregoing, it is obviously desirable to be able to reduce all three mentioned types of impurities, oxide particles, trapped gas, and chemical impurities such as calcium, sodium, magnesium, and lithium and the like, in a continuous process and at a single station or operation. It is also highly desirable that any such process be compatible with existing level pour molten metal transfer systems. As is known, aluminum's affinity for oxygen has fostered widespread use in the aluminum industry of substantially horizontally level molten metal transfer systems to avoid the turbulence and surface agitation, and resulting oxide formation, which could be encountered if the metal were permitted to drop significant heights during transfer.